There have been proposed various methods for producing lithium hexafluorophosphate, which is an electrolyte useful for lithium ion batteries, etc. In methods for producing lithium hexafluorophosphate using a solvent, there is a method (Non-patent Publication 1) in which a gaseous phosphorus pentafluoride is reacted with lithium fluoride that is dissolved by using anhydrous hydrogen fluoride as a solvent, and then the resulting lithium hexafluorophosphate is crystallized and taken out.
In this method, reaction yield of lithium hexafluorophosphate is high. It is, however, necessary to use as a solvent a large amount of anhydrous hydrogen fluoride, which is high in vapor pressure and has toxicity and corrosiveness. Therefore, its handling is not easy. Furthermore, there are many factors leading to the increase of cost, such as the necessity of producing phosphorus pentafluoride as one of the raw materials in a separate process, the necessity of having a crystallization process of lithium hexafluorophosphate, etc.
In common electrolyte solution productions, there is conducted a method in which lithium hexafluorophosphate is firstly produced, and then it is dissolved in a predetermined solvent for lithium batteries to make an electrolyte solution. As to the method of producing lithium hexafluorophosphate, for example, there is a method (Patent Publication 1) in which a solid lithium fluoride is reacted with a gaseous phosphorus pentafluoride with no solvent. In this method, there is a risk in which a film of the reaction product is formed on the surface of lithium fluoride, thereby the reaction does not proceed completely, and the unreacted lithium fluoride remains.
Similarly, there is also a method (Patent Publication 2) in which a reaction is conducted by adding anhydrous hydrogen fluoride to phosphorus pentachloride and lithium fluoride with no solvent. In this method, it is not easy to control the reaction, and therefore it is necessary to conduct a cooling until tens of degrees Celsius below freezing point.
Furthermore, there is a method (Patent Publication 3) in which lithium fluoride is reacted with phosphorus pentafluoride in an organic solvent. This method has great advantages in terms of reaction control and purity of the reaction product. It is, however, necessary to produce and handle phosphorus pentafluoride as one of the raw materials in a separate process, as mentioned above. Therefore, a problem of the cost remains.
Furthermore, there is also a method of producing lithium hexafluorophosphate (Patent Publication 4) in which, while anhydrous hydrogen fluoride or a polar organic solvent of CH3CN is used as a solvent, phosphorus trichloride, chlorine and hydrogen fluoride are reacted together to obtain phosphorus pentafluoride, followed by adding lithium fluoride to the same reactor to conduct a reaction with phosphorus pentafluoride. This method is efficient since the production of phosphorus pentafluoride is also conducted in the same reactor. It, however, goes through the formation of phosphorus pentafluoride, which is high in vapor pressure. Therefore, it is necessary to have expensive facilities such as a pressurized reactor and complicated operations. Furthermore, it is basically necessary to have a crystallization process. Therefore, it is difficult to fundamentally lower the cost of the electrolyte solution production. In this way, there remain many problems.
On the other hand, there is a method (Patent Publication 5), in which phosphorus trichloride, chlorine and lithium chloride are reacted together in a nonaqueous organic solvent, and then the reaction product formed in the solvent is reacted with hydrogen fluoride to produce an electrolyte solution for lithium ion batteries. In this method, it is possible to obtain a high-purity, electrolyte solution for lithium ion batteries that contains lithium hexafluorophosphate as an electrolyte. In case that hydrogen fluoride becomes excessive, however, it becomes necessary to remove this. Therefore, lithium chloride is added again as a purifying agent to remove acid impurities containing excessive hydrogen fluoride. With this, it is possible to obtain a higher-purity, electrolyte solution for lithium ion batteries.